The field of data communications typically uses a device, such as a modem, to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables devices to communicate rapidly large amounts of data. Modems communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation and differentially encoding the information to reduce errors and improve throughput. The constellation can include both analog and digital information or often merely digital information.
In the above mentioned communications system, typically both digital data and an analog signal are transmitted. The data signal to be transmitted is represented by a sequence of data symbols, where each data symbol is associated with a particular N-dimensional signal point value taken from a signal space. Similarly, the analog signal, which is represented by a voice signal, is processed so that it is mapped into the N-dimensional signal space to provide a voice signal point. This voice signal point defines the magnitude and angle of a voice signal vector about the origin of the signal space. The data symbol and the voice vector are then added together to select a resultant N-dimensional signal point. These N-dimensional signal points are grouped into signal space constellations and then transmitted to a far-end modem.
Upon reception of the transmitted N-dimensional signal point, the receiver of the far-end modem detects the embedded data symbol and subtracts the data symbol from the received N-dimensional signal point to yield the voice signal vector. This voice signal vector is then used to recreate the voice signal.
In the above mentioned communications environment, a control modem is located at a telephone company central office location. Connected to the control modem via a conventional copper wire pair are a plurality of remote modems. The remote modems reside at a common location, such as a residence or a business. The communication technique between the control modem and the remote modems is generally half duplex. In order to establish a communications connection between the central office modem and any of the remote modems, a lengthy preamble, which allows the modems to synchronize, at the start of each message is required. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two modems at a time. In a multipoint environment, greater circuit efficiency is possible because of the ability to connect multiple modems to the same communication channel.
In a 4 wire communications environment, the control modem can transmit continuously so that outbound preambles (from central office to remote location) are unnecessary, thus assuring remote modem synchronization. In a 2 wire multipoint environment, as contemplated by the present invention, when a remote modem transmits, it disrupts the outbound control signal from the control modem making it impossible for all remote modems connected to the communication line to maintain synchronization with the control modem. It is desirable to allow all remote modems to continuously maintain synchronization with the control modem in order to significantly reduce the training preamble necessary to establish a communication path between a remote modem and the control modem.